His first Thorndyke novel, The Red Thumb Mark (1907), introduced readers to scientific crime detection, employing fingerprints, blood analysis, and medical evidence decades before such techniques became routine in real policing. Freeman is also widely credited with inventing the "inverted detective story," in which the criminal and the method of the crime are revealed at the outset, and the reader's pleasure lies in watching the detective reconstruct the case. This format debuted in The Singing Bone (1912).
Among his other notable works are The Eye of Osiris (1911), The Mystery of 31 New Inn (1912), A Silent Witness (1914), Helen Vardon's Confession (1922), and The D'Arblay Mystery (1926). A founding member of the Detection Club, Freeman left a lasting influence on the golden age of detective fiction.